The present invention relates generally to electroslag refining, and, more specifically, to electroslag refining of superalloys.
Electroslag refining is a process used to melt and refine a wide range of alloys for removing various impurities therefrom, U.S. Pat. No. 5,160,532, Benz et al., discloses a basic electroslag refining apparatus over which the present invention is an improvement. Typical alloys which may be effectively refined using electroslag refining include those based on nickel, cobalt, zirconium, titanium, or iron. The initial, unrefined alloys are typically provided in the form of an ingot which has various defects or impurities which are desired to be removed during the refining process to enhance metallurgical properties thereof including oxide cleanliness, for example.
In a conventional electroslag apparatus, the ingot is connected to a power supply and defines an electrode which is suitably suspended in a water cooled crucible containing a suitable slag corresponding with the specific alloy being refined. The slag is heated by passing an electric current from the electrode through the slag into the crucible, and is maintained at a suitable high temperature for melting the lower end of the ingot electrode. As the electrode melts, a refining action takes place with oxide inclusions in the ingot melt being exposed to the liquid slag and dissolved therein, Droplets of the ingot melt, fall through the slag by gravity, and are collected in a liquid melt pool at the bottom of the crucible. The slag, therefore, effectively removes various impurities from the melt to effect refining thereof.
The refined melt may be extracted from the crucible by a conventional induction-heated, segmented, water-cooled copper guide tube. The refined melt extracted from the crucible in this manner provides an ideal liquid metal source for various solidification processes including, for example, powder atomization, spray deposition, investment casting, melt-spinning, strip casting, and slab casting.
In the exemplary electroslag apparatus introduced above, the crucible is conventionally water-cooled to form a solid slag skull on the surface thereof for bounding the liquid slag and preventing damage to the crucible itself as well as preventing contamination of the ingot melt from contact with the parent material of the crucible, which is typically copper. The bottom of the crucible typically includes a water-cooled, copper cold hearth against which a solid skull of the refined melt forms for maintaining the purity of the collected melt at the bottom of the crucible. A discharge guide tube below the hearth is also typically made of copper and is segmented and water-cooled for also allowing the formation of a solid skull of the refined melt for maintaining the purity of the melt as it is extracted from the crucible.
A plurality of water-cooled induction heating electrical conduits surround the guide tube for inductively heating the melt thereabove for controlling the discharge flow rate of the melt through the tube. In this way, the thickness of the skull formed around the discharge orifice in the guide tube may be controlled and suitably matched with melting of the ingot for obtaining a substantially steady state production of refined melt which is drained by gravity through the guide tube.
In order to achieve steady state operation of the electroslag refining apparatus, the apparatus must be suitably started without introducing undesirable contamination or impurities. In a conventional cold start method, a solid starter plate is fixed into position at the bottom of the crucible and above the discharge guide tube. The starter plate is formed of the same material as the ingot electrode except that it has been pre-refined and suitably machined for integral assembly into the electroslag refining apparatus. It is therefore relatively expensive and introduces additional complexity to the overall apparatus.
In order to effect a cold start, the electrode is positioned closely atop the starter plate, and conventional slag in particulate form is deposited atop the starter plate around the electrode. An electrical current is passed through the electrode to the starter plate and then through the atmosphere to cause an electrical arc to jump therebetween. The heat from the arc melts the surrounding solid slag. When sufficient slag is melted, the electrode is lowered into the slag to extinguish the arc, at which time power to the electrode effects direct resistance heating of the slag pool for increasing its temperature.
The heated slag pool then continues to melt the tip of the electrode and the starter plate until a hole is melted through the starter plate and liquid metal fills the crucible atop the guide tube. The hole through the starter plate enlarges until it reaches the outer perimeter of the plate, and resulting refined metal and slag skulls line the crucible and the guide tube. Steady state operation is reached when the rate of melting of the electrode and discharge flowrate from the guide tube are substantially equal.
Although the starter plate is initially formed of pre-refined metal, the electric arcing cold start introduces undesirable nitrides therefrom which are not removed by the electroslag refining process. If the starter plate melts too early during the startup process, unrefined metal or slag may flow through the guide tube causing undesirable impurities in the discharge stream.
Alternatively, the electroslag refining apparatus may be brought to steady state operation using a conventional hot start. In this procedure, slag is melted in an external furnace and deposited into the electroslag refining crucible to allow immediate resistance heating of the slag and corresponding melting of the electrode. However, a starter plate is still required to avoid transfer of undesirable slag through the guide tube. Although the undesirable nitrides are not introduced, the system is relatively more complex and expensive in view of the required external furnace, and the requirements of the pre-refined and machined expensive starter plate.
Accordingly, it is desired to provide an improved electroslag refining apparatus and method for in situ hot start thereof.
An electroslag refining apparatus includes upper and lower integral crucibles, with the lower crucible having a drain. In situ hot start is effected by depositing in the lower crucible a pre-refined starter. The starter is melted in the lower crucible to form a starter pool, and slag is deposited atop the starter pool for being melted thereby to develop a slag pool thereatop. An ingot electrode is lowered through the upper crucible to immerse a tip thereof into the slag pool. The electrode is powered to effect resistance heating of the slag pool to melt the electrode tip. The slag and starter pools are increased in volume into the upper crucible, with the drain then being opened to effect steady state operation.